This invention relates to Schmitt trigger circuits and, more particularly, to a Schmitt trigger adapted to receive a drive signal from circuitry based upon different transistor architecture.
A Schmitt trigger is a well-known binary circuit that exhibits hysteresis. Thus, a Schmitt trigger changes state at a high threshold level responsive to an increasing signal input and changes state at a lower threshold level responsive to a decreasing signal input. The hysteresis characteristic enables the Schmitt trigger to reject noise effectively, while reshaping a binary signal applied to its input. For this reason, Schmitt triggers are commonly used as the input stages of an integrated circuit chip to receive binary drive signals transmitted from the outputs of another integrated circuit chip. In this setting, a Schmitt trigger must sometimes process binary signals generated by a different transistor architecture. For example, the output driver stage of a chip may be configured in bipolar TTL circuitry to provide the high power level necessary to transmit binary signals from chip to chip, while the input receiving stages and the data processing circuitry on the chip may be configured in CMOS (complementary metal oxide semiconductor) architecture to reduce the power consumption on the chip. Such an interface of different computer architectures may present to the input Schmitt trigger stages of a chip an incompatible drive signal swing that leads to large power consumption. For example, a typical nominal signal swing in TTL circuitry is 0 to 3.5 volts, with a worse case signal swing of 0.8 to 2.0 volts, while CMOS transistor architecture is typically designed to operate on a drive signal swing of 0 to 5.0 volts. In such case, the TTL drive signal may not turn the CMOS transistors completely off because of its relatively small signal swing.